If you're going for a Single Action, i.e. a Colt Style, make the barrel either 7 1/2" or 4 3/4", traditional SA barrel lengths. A 6" barrel just doesn't look right on a SA revolver.
Make it a flat top similar to a Colt New Frontier and give it decent sights. Also, provide a wide, smooth trigger.
Depending on the size of your frame, consider calibers .32-20 or .357 Magnum. If you're going for big bore, .44 Magnum, .45 Colt or .41 Magnum.
And, of all things, don't do anything stupid like trying to make it opposite handed from traditional SA designs. There was a revolver made a few years ago, the Texas Longhorn, which was a beauty of a gun and very well made, but the maker put the loading gate and ejector rod on the wrong side.
And above all, use carbon steel that can be blued. Stainless steel Single Action revolves are a blasphemy to shootists!
Them's my thoughts.
Bob Wright